Microbial growth and metabolism:
In Medical Microbiology, microbial cultivation is required
for the following purposes:
In Medicine:
1-Diagnosis of most infectious diseases.
2-Selection of drug of choice ( antibiotics ) for treatment of
infection.
In other fields:
3-Preparation of Vaccine.
4-Research tool in molecular genetics.
Microbial growth requirements
Chemical growth factors
What are the chemical growth factors required for isolation of
microbes in vitro?
1-Carbon: Organic source:
Glucose
Inorganic source: CO2
2-Nitrogen: Organic source:
Protein
Inorganic source: atmospheric nitrogen
Autotrophic microbes using inorganic carbon and nitrogen.
Heterotrophic microbes using organic carbon and nitrogen.
Other chemical requirements
3- Hydrogen, Oxygen, Phosphorus , and Sulfur.
Physical growth factors:
1- pH and Buffer requirements:
Pathogenic bacteria grow best at neutral pH which is typically
between pH (6.8 to 7.4).
-Acidophilic bacterium is able to survive with acidic pH.
Example: Helicobacter pylori
-On the other extreme, bacteria that prefer alkaline conditions are
known as alkaliphiles.
Example: Vibrio cholerae.
Fungi such as yeasts and molds; grow best at acidic conditions
( pH 5 ).
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2- Salts concentration:
A specific concentration of NaCl is required for microbial growth
in vitro.
It is equal to normal saline salts concentration (0.9% NaCl).
Halophilic bacteria resist high salt concentration.
3- Temperature requirements:
Mesophiles : Grow at optimum of 37 ᵒC.
Normal flora and most pathogens grow at or near human
body temp. of 37 ᵒC .
4- Gaseous requirements and Humidity :
According to oxygen requirements, microbes can be divided
into the following groups:
1- Strict ,Obligate Aerobe: O2 must be present.
Example: Mycobacterium.
2- Strict or obligate Anaerobes:
Molecular oxygen must be absent.
Example: Clostridium.
3- Facultative anaerobes:
Adaptable organisms that use oxygen when present but can switch to
anaerobic pathways in its absence. Example: Escherichia coli.
4- Microaerophilic:
Only use low concentrations of oxygen ( around 5%).
Example : Helicobacter pylori.
The Bacterial Growth Curve:
Bacterial growth is the division of one bacterium into two
daughter cells in a process called binary fission.
Bacterial growth curve represents the relationship between
microbial quantity and time of incubation.
During lag phase, bacteria adapt themselves to growth
conditions (number of dividing cell is zero).
Exponential phase ( Log phase): is a period characterized by
Rapid cell doubling.
Doubling time can be as short as twenty minutes or as long as several
days.
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During stationary phase, the growth rate slows as a result of nutrient
depletion and accumulation of toxic products.
This phase is a constant value as the rate of bacterial growth is equal
to the rate of bacterial death.
At death phase, bacteria run out of nutrients and die.
Microbial Metabolism:
According to biochemical pathway used in energy production,
bacterial metabolism can be categorized into three types:
1-Aerobic Respiration:
-Molecular oxygen serves as the final electron acceptor.
- 38 ATP molecules will be produced by oxidation of one
glucose molecule.
- Used by obligatory aerobic bacteria for energy production;
such as: Mycobacterium.
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2-Anaerobic Respiration:
-Inorganic sulfate or nitrate act as the final electron acceptor.
-38 ATP molecules will be produced by catabolism of one
glucose molecule.
-Used by obligatory anaerobic bacteria such as: Clostridium.
3-Fermentation:
-Lactic acid ( produced by bacteria) or ethanol ( produced by
yeast) serves as final electron acceptor.
-Only 2 ATP molecules will be produced by fermentation of
one glucose molecule.
-Used by facultative anaerobic bacteria such as : E. coli.
Microbial Genetics:
Prokaryotic Genome:
-Most prokaryotic genes are carried on the bacterial chromosome,
a single circle of DNA.
-Many bacteria contain additional genes on plasmids.
-Plasmid is an extra-circular supercoiled DNA that carry some
important genes such as the antibiotics resistance genes.
Both bacteria chromosome
and plasmid are
called replicons.
Gene Transfer:
-Transfer of DNA among prokaryotes is widespread between
different strains of same bacterial species.
Mechanisms of Gene Transfer:
1- Conjugation.
2- Transduction (is a phage-mediated genetic transfer).
3- Transformation.
Conjugation: is a mechanism of gene transfer by which
plasmids will be transferred from one bacterial cell to another
by a mean of Sex pili.
Hospital-dwelling bacteria resist antibiotics due to conjugation.
Conjugation:
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The Microbial Virulence factors:
Virulence factors are external cellular structures, enzymes, and
toxins that enhance microbial pathogenicity.
In general, the most important virulence factors are:
1-Microbial capsule:
Microbe resist host acidic environment (stomach gastric acid).
Microbe resist host proteolytic enzyme,
(Present in Saliva, and stomach).
Microbe resist phagocytosis.
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2-Fimbriae or Pili:
-Microbial adhesion to the host cell surface.
-Adhesion could be also enhanced by receptor-antigen
interaction.
3-Microbial Enzymes:
Collagenase enzyme enhances microbial invasion; due
to degradation of extracellular matrix components.
Urease: Neutralization of acidic pH ( urine, stomach).
Coagulase, Catalase have different functions.
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4-Bacterial Toxins:
A-Exotoxins:
- Well known poisonous substances.
- Chemical nature: Proteins (two polypeptide components).
- Almost all are Heat-labile at 60 ˚C.
- Intracellular toxin fraction could:
1-inhibit cellular protein biosynthesis.
2-Cause ionic imbalance and loss of water.
3-inhibit the release of neurotransmitters.
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B-Endotoxins:
-Chemical nature: Lipopolysaccharide, the component of
Gram’s negative bacterial outer membrane.
-Heat-Stable at 100 ˚C.
5-The microbial Hemolysin:
-Degradation of RBCs, Hemoglobin and NADH will be released.
6-The microbial Haemagglutinine and Coagulase enzyme:
-Agglutination of RBCs; the microbe escapes Humoral
immunity.
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7-The microbial Beta-Lactamases:
-Some microbes have ability to resist antibiotics due to
production of Beta-Lactamase enzymes.
Some strains of
Staphylococci can
hydrolyze the BetaLactam ring shown.